Theoretically predicted properties of type II radio emission from an interplanetary foreshock

Abstract

We investigate the causes of variations in interplanetary type II radio bursts using an analytic model which predicts the emission generated by electron beams in the foreshock regions upstream of an interplanetary shock wave. Trends in source region characteristics and remotely observed radio fluxes are calculated as a function of a number of solar wind and shock parameters. Calculations are performed for a single three‐dimensional ripple on the global shock surface. Radio‐loud ripples are predicted to have larger shock speeds relative to the solar wind speed, higher levels of nonthermal electrons, larger radii of curvature, and be moving through higher density regions than radio‐quiet ripples. These predictions are qualitatively consistent with available observations. The predicted emission depends most sensitively on the speed of the shock relative to the solar wind. Strong correlations are found between the intensity of fundamental emission and the level of nonthermal electrons present in the tail of the incident solar wind electron distribution. Harmonic emission is found to be most sensitive to variations in the electron temperature T e of the incident solar wind. These results indicate that the bursty nature of typical type II observations can be accounted for by a shock propagating through an inhomogeneous solar wind.